Developmental stress: evidence for positive phenotypic and fitness effects in birds

The developmental environment has strong and pervasive effects on animal phenotype. Exposure to stress during development (in the form of elevated glucocorticoid hormones or food restriction) is one environmental cue that can have strong formative effects on morphology, physiology, and behavior. Although many of the effects of developmental stress appear negative, there is increasing evidence for an adaptive role of developmental stress in shaping animal phenotype. Here, we take a three-pronged approach to review studies that have uncovered positive effects of developmental stress on phenotype in birds. We focus on studies that: (1) examine phenotypic effects likely to increase fitness in offspring, (2) directly identify increased fitness in offspring, or (3) provide evidence of fitness benefits to the mother, at a cost to the offspring. Throughout, we focus on studies that evaluate the environment when assessing the 'costs/benefits' of phenotype alterations and examine the effects of developmental stress across life-history stages. Finally, we consider the two common methods used to simulate developmental stress: food restriction and direct hormone manipulation. Although these methods are often considered to elicit equivalent responses, there has been very little discussion of this in the literature. To this end, we review the main methods used to implement developmental stress in experimental studies and discuss how they may simulate different environmental conditions. In light of our conclusions, we propose possible avenues for future research, stressing the need for a greater focus on direct fitness metrics, longitudinal studies, and experiments in free-living animals.